the present version of man, first appeared, he was in evidence something like 40,000 to 50,000 years ago. It has been estimated that for the some 600,000 years of the Paleolithic Age (the old Stone Age) population growth perhaps approximated 0.02 per 1,000 per year (3). During the 3 centuries of the modern era population growth increased from about 3 per 1,000 to 10 per 1,000 per year between World Wars I and II. The rate of world population growth continued to accelerate after World War II, so that in 1965 it approximated 20 per 1,000 per year. In the course of man's inhabitation of this globe, then, his rate of population growth has increased from a rate of about 2 percent per millennium to 2 percent per annum, a thousandfold increase in growth rate.
From time to time there has been interest in the answer to the question, "How many people have lived on the earth?" Estimates in an effort to answer this question vary somewhat, but a good answer is about 69 billion up to 1960 (4). This would mean that of the total number of people ever born up to 1960, some 4 percent were living in 1960.
Population data prior to the modern era are admittedly speculative. But they provide a reasonably sound perspective and permit a very firm conclusion: Whatever his precise numbers may have been, during his habitation of this planet man has experienced a great increase in his rate of growth.
This conclusion is supported by placing in perspective the present rate of world population increase, estimated by the United Nations as approximately 2 percent per year. Although 2 percent per year may seem like a small re'turn on investment, it is a tremendous rate for world population growth. For example, to produce a population of 3.6 billion world population in 1969, one dozen persons increasing at a rate of 2 percent per year would have required only 976 years. Yet Homo sapiens alone has been on this earth at least 40,000 to 50,000 years. Similarly, the same one dozen persons reproducing at the rate of 2 percent per year since the year A.D. 1 could by 1965 have had over 400 million descendants for each person actually present in 1965.
Further appreciation of the meaning of a 2 percent rate of increase per year is gained by observing the population that this growth rate would produce in the future. In about 650 years there would be one person for each square foot of land surface on the globe, including mountains, deserts, and the arctic wastes. It would generate a population which would weigh as much as the earth itself in 1,566 years. These periods of time may seem long when measured by the length of an individual lifetime. But they are but small intervals in the time perspective of the evolutionary development of man.
It is possible to summarize quickly the remarkable acceleration which man has experienced in his growth rate. Between 1650 and 1750, at an annual rate of growth of 0.3 percent, world population would have required 231 years to double. Between 1850 and 1900, the growth rate had risen to 0.6 percent per